The present invention relates to X-ray imaging apparatus (X-ray image pickup device) and more particularly, to a technology effectively applicable to multilayer detector type X-ray tomograph and method for reconstructing a tomogram of a subject from X-ray images detected by helically rotating an X-ray generating source and a multilayer X-ray detector around the subject.
A conventional multilayer detector type X-ray tomograph is comprised of a scan driver including an X-ray generating source and a multilayer detector, a bed including a mechanism for supporting a subject and moving it along a body-axis direction of the subject, control means for controlling the scan driver and the bed, data collection means for collecting projection data detected by the multilayer detector, reconstruction operation means for reconstructing X-ray absorption coefficient distribution of the subject from the collected projection data, display means for imaging and displaying the reconstructed X-ray absorption coefficient distribution, and a console for an operator to set values necessary for the multilayer detector type X-ray tomograph or to input commands.
Hereinafter, in the present specification hereinafter, the multilayer detector means has the meaning of a detector in which many X-ray detection elements are arranged along the rotation direction and in the rotary axis direction of the scan driver that is, in the form of a matrix which forms part of the cylindrical surface. Most X-ray tomograph put into practice at present have been so-called single-layer detector type X-ray tomograph, which has many detection elements arranged along the rotation direction on an arc. Contrarily, the multilayer detector has detectors of this type (single-layer) stacked to form layers also in the rotary axis direction and is discriminated by being called the multilayer type.
Next, a conventional imaging method by the multilayer detector type X-ray tomograph will be, described. In the multilayer detector type X-ray tomograph, too, an imaging method comparable to the method which has hitherto been known as the helical scan method is employed. More particularly, the scan driver is continuously rotated and at the same time, the bed carrying a subject is moved in the rotary axis direction; and during this operation, X-rays are projected from the X-ray generating source and X-rays transmitting through the subject are measured as projection data (X-ray images) by means of the multilayer detector. In other words, the X-ray generating source and the multilayer detector moved on orbits which are helical relative to the subject. The moving speed (moving distance per one rotation of the scan driver) of the bed and the aperture width of the detection element in the rotary axis direction (moving direction of bed) at that time were important parameters for determining spatial resolution of a reconstructed image in the rotary axis direction.
On the other hand, in the conventional single-layer detector type X-ray tomograph, the scan was carried out usually by making the moving speed of the bed nearly equal to the aperture width of the detection element in the rotary axis direction. Therefore, if the aperture width of the detection element in the rotary axis direction remains unchanged, spatial resolution nearly equal to that in usual imaging method using the single-layer detector type X-ray tomograph could be obtained with the multilayer detector type X-ray tomograph even when the moving speed of the bed approximates [the aperture width]xc3x97[the number of detector layers]. Accordingly, by forming the detector to the multilayer type, throughput could be improved.
As a reconstruction operation method of reconstructing X-ray absorption coefficient distribution in the subject, that is, a reconstructed image from projection data measured as above by the multilayer detector type X-ray tomograph, various operation methods have already been proposed and as a most fundamental multilayer detector type X-ray tomograph, an apparatus described in, for example, JP-A-4-343836 (hereinafter referred to as xe2x80x9cliterature 1xe2x80x9d) may be referred to. In the multilayer detector type X-ray tomograph described in the literature 1, projection data measured by the multilayer detector are approximately considered as projection data measured by detectors stacked up single-layer to effect reconstruction. Measurement using the multilayer detector greatly differs from that based on the single-layer detector in that projection is slanted. Namely, in the measurement using the single-layer detector, all projected X-ray beams passed through a plane containing the X-ray generating source 1 and being vertical to the rotary axis. On the other hand, in the measurement using the multilayer detector, most projections were not vertical to the rotary axis but were oblique thereto. Therefore, the more the number of layers of the detector and the larger the arrangement pitch between detector layers in the rotary axis direction, the larger the inclination becomes.
In the reconstruction operation method described in the literature 1, however, this inclination of projection was not taken into consideration. Therefore, as far as the conventional reconstruction operation method was used, actually measured X-lay beam transmitting paths differed from X-ray beam transmitting paths presupposed approximately for the reconstruction operation and as a result, there arose a problem that, for example, a partial volume artifact is generated to degrade the quality of a reconstructed image. As a method for solving the above problem, a multilayer detector type X-ray tomograph using a reconstruction operation method described in, for example, JP-A-8-322831 (hereinafter referred to as xe2x80x9cliterature 2xe2x80x9d) has been available. In the reconstruction operation method in the apparatus, the two closest X-ray beams were determined in respect of individual pixels on a reconstruction cross section and weighting complying with distances between these beams and the reconstrucntion cross section was effected to conduct subsequent inverse projection. In this manner, in the reconstruction operation method described in the literature 2, the reconstruction operation taking the inclination of projection into account was performed to thereby obtain a reconstructed image of good picture quality.
The present inventor has studied the prior arts to find the following problems. Even in the conventional multilayer detector type X-ray tomograph described in the literature 2, there arose as below. A first problem resided in the amount of operations necessary for determining the two closest X-ray beams in respect of the individual pixels on the reconstruction cross section and a second problem resided in continuity of projection.
Firstly, the first problem will be described. In utilizing the multilayer detector type X-ray tomograph, convenience will be met by the ability to suitably select the moving speed of the bed and the aperture width of the detector in the rotary axis direction depending on the condition of a subject, a region of interest in diagnosis or sought image quality or spatial resolution. Then, the combination of the two closest X-ray beams in respect of the individual pixels on the reconstruction cross section changes in various ways depending on the moving speed of the bed and the aperture width of the detector in the rotary axis direction. This combination may be calculated every operation or may be calculated in advance and held in a table. But in the former case, a problem of a drastic increase in computational amount arose. On the other hand, the latter case faced a problem of a drastic increase in table capacity.
Next, the second problem will be described. As described above, when the two closest X-ray beams were determined in respect of the individual pixels on the reconstruction cross section, there arose a problem that continuity of projection data to be inversely projected between pixels is impaired. For example, it is assumed that the two closest X-ray beams in respect of predetermined pixels are for example, an X-ray beam contained in a detector layer N1 for projection P1 and an X-ray beam contained in a detector layer N2 for projection P2. Contrarily, there is a possibility that the two closest X-ray beams in respect of pixels directly adjoining those pixels are an X-ray beam contained in a detector layer N1xe2x80x2 (N1xe2x80x2xe2x89xa0N1) for projection P1xe2x80x2(P1xe2x80x2xe2x89xa0P1) and an X-ray beam contained in a detector layer N2xe2x80x2(N2xe2x80x2xe2x89xa0N2) for projection P2xe2x80x2(P2xe2x80x2xe2x89xa0P2). In other word, although the former pixels and the latter pixels are adjacent to each other, the X-ray be ms to be inversely projected belong to totally different projections. This does not seems to be of a serious problem when considering that the X-ray absorption coefficient distribution in the subject changes spatially continuously but actually, because of the presence of non-uniform sensitivity of individual detector layers and offset of incident X-rays in the rotary axis direction due to the influence of characteristics of the X-ray generating source and scattered rays, the aforementioned degradation in the continuity of X-ray beams has an influence upon image quality which is not negligible. As a result, there arose a problem that a fine streak artifact is generated on a reconstructed image to degrade the image quality of the reconstructed image.
An object of the present invention is to provide an X-ray imaging apparatus which can reconstruct a tomogram of high image quality from X-ray projection data obtained by helically scanning the periphery of a subject. Another object of the present invention is to provide an X-ray imaging apparatus capable of improving the accuracy of diagnosis by an inspector and to provide an X-ray imaging apparatus capable of reducing the amount of operations when a tomogram is reconstructed from X-ray images obtained by helically scanning the periphery of the subject. The objects and novel features of the present invention will become clear from a description of the present specification and the accompanying drawings. Typical inventions disclosed in the present application will be outlined as below.
(1) In an X-ray imaging apparatus having an X-ray generating source for irradiating X-ray beams on a subject, multilayer X-ray imaging means having two or more juxtaposed detector arrays each having X-ray detector cells 11 arranged in line and being disposed to oppose the X-ray generating source so as to measure X-ray images of the subject, rotation means for rotating an imaging system comprised of the X-ray generating source and the multilayer X-ray detecting means around the subject, moving means for relatively moving the imaging system and the subject in a body axis direction of the subject, reconstructing means for reconstructing a tomogram of the subject at a position specified by an inspector on the basis of X-ray images, and display means for displaying the tomogram, the reconstructing means comprises selecting means for selecting, from X-ray images measured from the 360xc2x0 periphery of the subject at a cross-sectional position designated by the inspector, X-ray images measured within a range; of xe2x88x92360xc2x0 to +360xc2x0 distant from a rotation angle of the imaging system at the cross-sectional position, and inter polation operation means for interpolating projection values of X-ray images in respect of individual pixels at the cross-sectional position specified by the inspector from distances in the moving direction of the moving means (bed) between positions of the selected X-ray images (selected images) and the cross-sectional position specified by the inspector and the selected X-ray images, whereby the tomogram at the cross-sectional position is reconstructed from an interpolated image.
(2) In an X-ray imaging method used in the X-ray imaging apparatus as recited in (1), the step of reconstructing a tomogram of the subject executed by the reconstructing means comprises a first step of selecting, from X-ray images measured from the 360xc2x0 periphery of the subject at the cross-sectional position specified by the inspector, X-ray images within a range of xe2x88x92360xc2x0 to +360xc2x0 distant from a rotation angle of the imaging system at the cross-sectional position, a second step of performing an interpolation operation for interpolating projection values of X-ray images in respect of individual pixels at the cross-sectional position by using distances in the moving direction of the moving means between positions of the selected X-ray images and the cross-sectional position and the selected X-ray images, and a third step of re constructing a tomogram at the cross-sectional position from interpolated projection values, and further, the first step includes selecting, from the X-ray images measured within the range of xe2x88x92360xc2x0 to +360xc2x0 distant from the rotation angle of the imaging system the cross-sectional position, X-ray images measure at positions where projection directions of X-ray beams are identical, (b) the first step includes selecting, from the X-ray images measured within the range of xe2x88x92360xc2x0 to +360xc2x0 distant from the rotation angle of the imaging system at the cross-sectional position, X-ray images measured at positions where projection directions of X-ray beams oppose to each other and the step of performing reconstruction has the step of correcting blur in measured X-ray images wherein the second, step includes interpolating projection values of X-ray images in respect of the individual pixels at the cross-sectional position by using corrected X-ray images, and (c) the first step includes selecting X-ray images measured by all X-ray detector cells arranged in the moving direction and the second step includes interpolating projection values of X-ray images in respect of the individual pixels at the cross-sectional position by using distances in the moving direction between the selected X-ray images and the cross-sectional position specified by the inspector and the selected X-ray images.
According to the aforementioned (1) and (2), when the interpolation operation means operates projection values in respect of the individual pixels at the cross section specified by the inspector, two sets of projection data for one rotation picked up at positions near the reconstruction cross-sectional position are selected and when projection values corresponding to pixels in respect of the individual pixels at the reconstruction cross section are calculated, projection data corresponding to predetermined X-ray beams at individual projection angles are used, the projection values can be calculated without impairing continuity of projection values to be inversely projected between pixels. Accordingly, the generation of, for example, a fine streak artifact can be prevented and as a result the image quality of the tomogram can be improved. Consequently, the efficiency of diagnosis by a doctor representing the inspector can be improved and the diagnostic accuracy can also be improved.
Further, since the projection values for the individual pixels are calculated by reflecting the distances between the X-ray beams and the reconstruction cross section, the generation of, for example, a partial volume artifact can be prevented and as a results the image quality of the tomogram can further be improved. Consequently, the efficiency of diagnosis by a doctor representing the inspector can further be improved and also, the diagnostic accuracy can further be improved. Besides, X-ray beams to be subjected to interpolation can be set easily in advance regardless of the positions of the pixels on the reconstruction cross section and hence, the amount of operations necessary for interpolation operation can be reduced. Accordingly, time required for reconstruction of a tomogram an be shortened to further improve the efficiency of diagnosis by the doctor representing the inspector.